Positive electrode active material for nonaqueous electrolyte secondary battery, method for producing same, and nonaqueous electrolyte secondary battery using said positive electrode active material

ABSTRACT

Provided is a method for producing a positive electrode active material for nonaqueous electrolyte secondary batteries, the method including: a mixing step of obtaining a W-containing mixture of Li metal composite oxide particles represented by the formula: LizNi1-x-yCOxMyO2 and composed of primary particles and secondary particles formed by aggregation of the primary particles, 2 mass % or more of water with respect to the oxide particles, and a W compound or a W compound and a Li compound, the W-containing mixture having a molar ratio of the total amount of Li contained in water and the solid W compound or the W compound and the Li compound of 3 to 5 with respect to the amount of W contained therein; and a heat treatment step of heating the W-containing mixture to form lithium tungstate on the surface of the primary particles of the Li metal composite oxide particles.

BACKGROUND 1. Field of the Invention

The present invention relates to a positive electrode active materialfor nonaqueous electrolyte secondary batteries and a production methodthereof, and a nonaqueous electrolyte secondary battery using thepositive electrode active material.

2. Description of the Related Art

In recent years, with the wide adoption of portable electronic devicessuch as mobile phones and laptop computers, the development of small andlightweight nonaqueous electrolyte secondary batteries having highenergy density is strongly desired. Further, the development of highpower secondary batteries as batteries for electric cars includinghybrid cars is strongly desired.

Examples of the secondary batteries satisfying such demands includelithium ion secondary batteries.

Such lithium ion secondary batteries are composed of a negativeelectrode, a positive electrode, an electrolyte, etc., and materialscapable of intercalation and deintercalation of lithium ions are usedfor the active materials of the negative electrode and the positiveelectrode. The lithium ion secondary batteries are now being activelystudied and developed. Above all, lithium ion secondary batteries usinga layered or spinel lithium-metal composite oxide as a positiveelectrode material allow a high voltage of 4-V class to be obtained, andtherefore are being put into practical use as batteries having highenergy density.

Main examples of materials proposed so far include lithium cobaltcomposite oxide (LiCoO₂) that is comparatively easily synthesized,lithium nickel composite oxide (LiNiO₂) using nickel that is lessexpensive than cobalt, lithium nickel cobalt manganese composite oxide(LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂), and lithium manganese composite oxide(LiMn₂O₄) using manganese.

Among these, lithium nickel composite oxide is gaining attention as amaterial that allows high battery capacity to be obtained. Further, aresistance reduction that is necessary for power enhancement is regardedas being important in recent years.

As a method for achieving the aforementioned resistance reduction,addition of different elements is used, and transition metals capable ofhaving high valence such as W, Mo, Nb, Ta, and Re are considered to beuseful, in particular.

For example, Japanese Patent Laid-Open No. 2009-289726 proposes alithium transition metal compound powder for lithium secondary batterypositive electrode materials containing one or more elements selectedfrom Mo, W, Nb, Ta, and Re in an amount of 0.1 to 5 mol % with respectto the total molar amount of Mn, Ni, and Co, where the total atomicratio of Mo, W, Nb, Ta, and Re with respect to the total of Li and themetal elements other than Mo, W, Nb, Ta, and Re on the surface portionsof primary particles is preferably 5 times or more the atomic ratio ofthe whole primary particles.

According to this proposal, the cost reduction, high safety, high loadcharacteristics, and improvement in powder handleability of the lithiumtransition metal compound powder for lithium secondary battery positiveelectrode materials can be achieved all together.

However, the aforementioned lithium transition metal compound powder isobtained by pulverizing a raw material in a liquid medium, spray dryinga slurry in which the pulverized materials are uniformly dispersed, andfiring the obtained spray-dried material. Therefore, some of differentelements such as Mo, W, Nb, Ta, and Re are substituted with Ni disposedin layers, resulting in a reduction in battery characteristics such asbattery capacity and cycle characteristics, which has been a problem.

Further, Japanese Patent Laid-Open No. 2005-251716 proposes a positiveelectrode active material for nonaqueous electrolyte secondary batterieshaving at least a lithium transition metal composite oxide with alayered structure, wherein the lithium transition metal composite oxideis present in the form of particles composed of either or both ofprimary particles and secondary particles as aggregates of the primaryparticles, and wherein the particles have a compound including at leastone selected from the group consisting of molybdenum, vanadium,tungsten, boron, and fluorine at least on the surface.

With that, it is claimed that the positive electrode active material fornonaqueous electrolyte secondary batteries having excellent batterycharacteristics even in more severe use environment is obtained, andthat the initial characteristics are improved without impairing theimprovement in thermostability, load characteristics, and outputcharacteristics particularly by having the compound including at leastone selected from the group consisting of molybdenum, vanadium,tungsten, boron, and fluorine on the surface of the particles.

However, the effect by adding the at least one element selected from thegroup consisting of molybdenum, vanadium, tungsten, boron, and fluorineis to improve the initial characteristics, that is, the initialdischarge capacity and the initial efficiency, where the outputcharacteristics are not mentioned. Further, according to the disclosedproduction method, the firing is performed while the additive element ismixed with a heat-treated hydroxide together with a lithium compound,and therefore the additive element is partially substituted with nickeldisposed in layers to cause a reduction in battery characteristics,which has been a problem.

Further, Japanese Patent Laid-Open No. H11-16566 proposes a positiveelectrode active material in which the circumference of the positiveelectrode active material is coated with a metal containing at least oneselected from Ti, Al, Sn, Bi, Cu, Si, Ga, W, Zr, B, and Mo and/or anintermetallic compound obtained by combining a plurality of theseelements, and/or an oxide.

It is claimed that such coating can ensure the safety by absorbingoxygen gas, but there is no disclosure on the output characteristics.Further, the disclosed production method involves coating using aplanetary ball mill, and such a coating method causes physical damage onthe positive electrode active material, resulting in a reduction inbattery characteristics.

Further, Japanese Patent Laid-Open No. 2010-40383 proposes a positiveelectrode active material heat-treated while a tungstate compound isdeposited on composite oxide particles mainly composed of lithiumnickelate and having a carbonate ion content of 0.15 weight % or less.

According to this proposal, since the tungstate compound or adecomposition product of the tungstate compound is present on thesurface of the positive electrode active material, and the oxidationactivity on the surface of the composite oxide particles during chargeis suppressed, gas generation due to the decomposition of the nonaqueouselectrolyte or the like can be suppressed, but there is no disclosure onthe output characteristics.

Further, the disclosed production method is to deposit a solution inwhich a sulfuric acid compound, a nitric acid compound, a boric acidcompound, or a phosphate compound serving as a deposition component isdissolved in a solvent together with the tungstate compound, on thecomposite oxide particles that are preferably heated to at least theboiling point of the solution in which the deposition component isdissolved, where the solvent is removed within a short time, andtherefore the tungsten compound is not sufficiently dispersed on thesurface of the composite oxide particles and is not uniformly deposited,which has been a problem.

Further, improvements in power enhancement by lithium nickel compositeoxide have been made. For example, Japanese Patent Laid-Open No.2013-125732 proposes a positive electrode active material for nonaqueouselectrolyte secondary batteries having fine particles containing lithiumtungstate represented by any one of Li2WO4, Li4WO5, and Li6W2O9 on thesurface of a lithium-metal composite oxide composed of primary particlesand secondary particles formed by aggregation of the primary particles,where high power is supposed to be obtained together with high capacity.Although the power is enhanced while the high capacity is maintained,further enhancement in capacity is required.

In view of such problems, it is an object of the present invention toprovide a positive electrode active material for nonaqueous electrolytesecondary batteries which allows higher power together with highcapacity to be obtained when used as a positive electrode material,while suppressing an increase in gas generation.

SUMMARY

As a result of diligent studies on the powder characteristics oflithium-metal composite oxide used as a positive electrode activematerial for nonaqueous electrolyte secondary batteries and the effectthereof on the output characteristics of the battery, for solving theaforementioned problems, the inventors have found that the positiveelectrode resistance of the positive electrode active material can bereduced and the output characteristics of the battery can be improved byforming lithium tungstate having a specific form on the surface of theprimary particles and the secondary particles constituting thelithium-metal composite oxide. Further, as a production method thereof,they have found that the form of lithium tungstate can be controlled byheat-treating a mixture controlled to have a specific ratio of lithiumand tungsten other than the lithium-metal composite oxide particles,thereby accomplishing the present invention.

More specifically, the first aspect of the present invention is a methodfor producing a positive electrode active material for nonaqueouselectrolyte secondary batteries, including: a mixing step of obtaining atungsten-containing mixture of a lithium-metal composite oxide powderrepresented by the general formula: Li_(z)Ni_(1-x-y)Co_(x)M_(y)O₂ (where0<x≤0.35, 0≤y≤0.35, and 0.95≤z≤1.30 are satisfied, and M is at least oneelement selected from Mn, V, Mg, Mo, Nb, Ti, and Al) and having alayered crystal structure constituted by primary particles and secondaryparticles formed by aggregation of the primary particles, 2 mass % ormore of water with respect to the lithium-metal composite oxide powder,and a tungsten compound or a tungsten compound and a lithium compound,the tungsten-containing mixture having a molar ratio of a total amountof lithium contained in the water and the tungsten compound as a solidcomponent, or in the water, and the tungsten compound and the lithiumcompound as a solid component of 3 to 5, with respect to an amount oftungsten contained therein; and a heat treatment step of heating theobtained tungsten-containing mixture to form lithium tungstate on asurface of the primary particles of the lithium-metal composite oxide.

The second aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first aspect, further including, prior to themixing step: a water washing step of washing the lithium-metal compositeoxide powder with water by mixing the lithium-metal composite oxidepowder with the water to form a slurry; and a solid-liquid separationstep of performing solid-liquid separation subsequently to the waterwashing step.

The third aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the second aspect, wherein the lithium-metalcomposite oxide powder is contained in the slurry at a concentration of200 to 5000 g per 1 L of water.

The fourth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the second aspect and the third aspect, whereinthe tungsten-containing mixture is obtained by adding the tungstencompound at least during the water washing step or after thesolid-liquid separation step.

The fifth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the fourth aspect, wherein the slurry is formedby mixing the lithium-metal composite oxide powder with an aqueoussolution of the tungsten compound in the water washing step.

The sixth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the fourth aspect, wherein the tungsten compoundis in powder form.

The seventh aspect of the present invention is the method for producinga positive electrode active material for nonaqueous electrolytesecondary batteries according to the first aspect to the sixth aspect,wherein the heat treatment is performed at 100 to 600° C.

The eighth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first aspect to the seventh aspect, whereinan amount of tungsten contained in the tungsten-containing mixture isadjusted to 0.05 to 2.0 at % with respect to the total number of atomsof Ni, Co, and M contained in the lithium-metal composite oxide powder.

The ninth aspect of the present invention is a positive electrode activematerial for nonaqueous electrolyte secondary batteries composed of alithium-metal composite oxide powder having a layered crystal structureconstituted by primary particles and secondary particles formed byaggregation of the primary particles, wherein the positive electrodeactive material is represented by the general formula:Li_(z)Ni_(1-x-y)Co_(x)M_(y)W_(a)O_(2+α) (where 0<x≤0.35, 0≤y≤0.35,0.95≤z≤1.30, 0<a≤0.03, and 0≤α≤0.15 are satisfied, and M is at least oneelement selected from Mn, V, Mg, Mo, Nb, Ti, and Al) and has lithiumtungstate on a surface of the primary particles of the lithium-metalcomposite oxide, and Li₄WO₅ is contained in the lithium tungstate at aproportion of 50 to 90 mol %.

The tenth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the ninth aspect, wherein lithium is contained in the lithiumcompound other than the lithium tungstate present on a surface of thelithium-metal composite oxide in an amount of 0.08 mass % or less withrespect to the total amount of the positive electrode active material.

The eleventh aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the ninth and the tenth aspect, wherein tungsten is contained in thelithium tungstate an amount of 0.05 to 2.0 at % as the number of W atomswith respect to the total number of atoms of Ni, Co, and M contained inthe lithium-metal composite oxide.

The twelfth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the ninth aspect to the eleventh aspect, wherein the lithiumtungstate is present on the surface of the primary particles of thelithium-metal composite oxide as fine particles having a particle sizeof 1 to 200 nm.

The thirteenth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the ninth aspect to the eleventh aspect, wherein the lithiumtungstate is present on the surface of the primary particles of thelithium-metal composite oxide as a coating film having a film thicknessof 1 to 150 nm.

The fourteenth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the ninth aspect to the eleventh aspect, wherein the lithiumtungstate is present on the surface of the primary particles of thelithium-metal composite oxide in both forms of fine particles having aparticle size of 1 to 200 nm and a coating film having a film thicknessof 1 to 150 nm.

The fifteenth aspect of the present invention is a nonaqueouselectrolyte secondary battery having a positive electrode including thepositive electrode active material for nonaqueous electrolyte secondarybatteries according to the ninth aspect to fourteenth aspect.

According to the present invention, a positive electrode active materialfor nonaqueous electrolyte secondary batteries capable of achieving highpower together with high capacity, while the gas generation is reducedwhen used as a positive electrode material of a battery is obtained.

Further, the production method is easy and suitable for production on anindustrial scale, and the industrial value thereof is exceptionallylarge.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of an equivalent circuit used formeasurement examples of impedance evaluation and analysis.

FIG. 2 is a schematic sectional view of a 2032-type coin battery 1 usedfor battery evaluation.

FIG. 3 is an example showing a cross-sectional observation result of apositive electrode active material obtained in an example using ascanning microscope.

FIG. 4 is a schematic illustration of a laminated cell 4 used forbattery evaluation.

FIG. 5 is a schematic illustration showing a method for evaluating a gasgeneration in which the laminated cell 4 is pressed by a hydraulic pressmachine PA.

DETAILED DESCRIPTION

Hereinafter, for the present invention, a positive electrode activematerial of the present invention will be first described, andthereafter a production method thereof and a nonaqueous electrolytesecondary battery using the positive electrode active material of thepresent invention will be described.

(1) Positive Electrode Active Material

The positive electrode active material for nonaqueous electrolytesecondary batteries of the present invention is composed of alithium-metal composite oxide having a layered crystal structureconstituted by primary particles and secondary particles formed byaggregation of the primary particles, wherein the positive electrodeactive material has a composition represented by the formula:Li_(z)Ni_(1-x-y)CO_(x)M_(y)W_(a)O_(2+α) (where 0<x≤0.35, 0≤y≤0.35,0.95≤z≤1.30, 0<a≤0.03, and 0≤α≤0.15 are satisfied, and M is at least oneelement selected from Mn, V, Mg, Mo, Nb, Ti, and Al) and has lithiumtungstate on surface of the primary particles of the lithium-metalcomposite oxide, and Li₄WO₅ is contained in the lithium tungstate at aproportion of 50 to 90 mol %.

In the present invention, high charge-discharge capacity is obtained byusing the lithium-metal composite oxide represented by the formulaLi_(z)Ni_(1-x-y)Co_(x)M_(y)O₂ (where 0<x≤0.35, 0≤y≤0.35, and 0.95≤z≤1.30are satisfied, and M is at least one element selected from Mn, V, Mg,Mo, Nb, Ti, and Al) as a base material. For obtaining highercharge-discharge capacity, x+y≤0.2 and 0.95≤z≤1.10 are preferablysatisfied in the aforementioned the formula. In the case where highthermal stability is required, x+y>0.2 is preferably satisfied.

Further, the base material is in the form of a lithium-metal compositeoxide powder constituted by primary particles and secondary particlesformed by aggregation of the primary particles (hereinafter, thesecondary particles and the primary particles existing alone may bereferred to collectively as “lithium-metal composite oxide particles”),and Li₄WO₅ is contained in the lithium tungstate formed on the surfaceof the primary particles (which will be hereinafter referred to as theproportion of Li₄WO₅) at a proportion of 50 to 90 mol %, therebyimproving the output characteristics while suppressing an increase ingas generation and maintaining the charge-discharge capacity.

Generally, when the surface of the positive electrode active material iscompletely coated with a different compound, the movement(intercalation) of lithium ions is significantly limited, and thereforehigh capacity that is an advantage of lithium nickel composite oxide iseventually offset.

In contrast, in the present invention, lithium tungstate is formed onthe surface of the lithium-metal composite oxide particles and thesurface of the primary particles thereinside, and the lithium tungstatehas high lithium ion conductivity and has an effect of promoting themovement of lithium ions. Therefore, the aforementioned lithiumtungstate is formed on the surface of the primary particles of thelithium-metal composite oxide particles, thereby forming Li conductionpaths at the interface with the electrolyte, so that the reactionresistance of the positive electrode active material (which may behereinafter referred to as positive electrode resistance) is reduced toimprove the output characteristics of the battery.

Thus, the reduction in positive electrode resistance reduces the voltageto be lost in the battery, and the voltage actually applied to the loadside is relatively increased, thereby allowing high power to beobtained. Further, the increase in the voltage applied to the load sideallows lithium to be sufficiently inserted into and removed from thepositive electrode, and therefore the charge-discharge capacity of thebattery (which may be hereinafter referred to as “battery capacity”) isalso improved.

Here, it is important to adjust the proportion of Li₄WO₅ in the lithiumtungstate to 50 to 90 mol %.

More specifically, the lithium tungstate may have a number of presenceforms such as Li₂WO₄, Li₄WO₅, and Li₆W₂O₉, among which Li₄WO₅ has highlithium ion conductivity, and the presence of Li₄WO on the surface ofthe primary particles reduces the reaction resistance of the positiveelectrode active material more significantly, thereby allowing a moresignificant effect of improving the output characteristics to beobtained. Further, the reduction in positive electrode resistance alsoenables the battery capacity to be improved.

However, when the lithium tungstate consists of Li₄WO₅ only, the gasgeneration when the battery is stored at high temperature increases, andtherefore a safety problem occurs. The details of the cause of theincrease in gas generation are unknown, but it is probably because Li ofLi₄WO₅ is easily dissociated in a solvent, particularly, water.

Accordingly, the proportion of Li₄WO₅ is set to 50 to 90 mol %,preferably 50 to 80 mol %, in the present invention, thereby allowingthe significant effect of reducing the reaction resistance to beobtained while suppressing the increase in gas generation.

Meanwhile, since Li₂WO₄ has high lithium ion conductivity, though it isnot as high as that of Li₄WO₅, and is less likely to be dissociated inwater, the effect of suppressing the increase in gas generation when thebattery is stored at high temperature battery is high.

Accordingly, it is preferable that the proportion of Li₂WO₄ contained inthe lithium tungstate formed on the surface of the primary particles(which will be hereinafter referred to as the proportion of Li₂WO₄) beset to 10 to 50 mol %, and the proportion of the total of Li₄WO₅ andLi₂WO₄ be set to 90 mol % or more. This situation allows the significanteffect of reducing the reaction resistance to be obtained, while furthersuppressing the increase in gas generation.

The form of the lithium tungstate may be determined by any method aslong as the form can be specified in terms of molar ratio and can bedetermined by an instrumental analysis using X-ray or electron beam.Further, it may be calculated by pH titration analysis usinghydrochloric acid.

Further, the contact with the electrolyte occurs on the surface of theprimary particles, and therefore it is important that the lithiumtungstate (which may be hereinafter referred to as “LWO”) be formed onthe surface of the primary particles. Here, the surface of the primaryparticles in the present invention includes the surface of the primaryparticles exposed on the outer surface of the secondary particles, andthe surface of the primary particles communicating with the outside ofthe secondary particles so as to allow the electrolyte to penetratetherethrough and exposed into voids in the vicinity of the surface ofthe secondary particles and inside thereof. Further, the surface of theprimary particles includes even the grain boundaries between the primaryparticles if the primary particles are not perfectly bonded, and theelectrolyte can penetrate therethrough.

The contact with the electrolyte occurs not only on the outer surface ofthe secondary particles formed by aggregation of the primary particlesbut also in the voids between the primary particles in the vicinity ofthe surface of the secondary particles and inside thereof and further atthe aforementioned imperfect grain boundaries, and therefore it isnecessary to form LWO also on the surface of the primary particles topromote the movement of lithium ions. Accordingly, the reactionresistance of the positive electrode active material can be furtherreduced by forming LWO more on the surface of the primary particleswhich can contact with the electrolyte.

Further, as the form of LWO on the surface of the primary particles,when the surface of the primary particles is coated with layeredmaterials, the contact area with the electrolyte is reduced. When suchlayered materials are formed, the formation of the compound tends toconcentrate on the surface of some specific primary particles. Since thelayered materials as coating materials have high lithium ionconductivity, the effects of improving the charge-discharge capacity andreducing the positive electrode resistance are obtained, but they arenot sufficient, leaving room for improvement.

Accordingly, for obtaining higher effect, LWO is preferably present onthe surface of the primary particles of the lithium-metal compositeoxide as fine particles having a particle size of 1 to 200 nm.

The contact area with the electrolyte is rendered sufficient by havingsuch a form, so that the lithium ion conductivity can be effectivelyimproved, thereby allowing the positive electrode resistance to be moreeffectively reduced and the charge-discharge capacity to be improved.When the particle size is less than 1 nm, the fine particles may fail tohave sufficient lithium ion conductivity in some cases. Further, whenthe particle size is over 200 nm, the formation of the fine particles onthe surface of the primary particles is made non-uniform, which mayresult in failure to obtain a higher effect of reducing the positiveelectrode resistance in some cases.

Here, the fine particles are not necessarily completely formed on theentire surface of the primary particles and may be scattered. Even whenscattered, the effect of reducing the reaction resistance of thepositive electrode is obtained as long as the fine particles are formedon the outer surface of the lithium-metal composite oxide particles andthe surface of the primary particles thereinside.

Further, not all of the fine particles are necessarily present as fineparticles having a particle size of 1 to 200 nm, and a high effect isobtained when 50% or more of the number of the fine particles formed onthe surface of the primary particles are preferably formed to have aparticle size in the range of 1 to 200 nm.

Meanwhile, when the surface of the primary particles is coated with athin film, Li conduction paths can be formed at the interface with theelectrolyte, while the reduction in specific surface area is suppressed,and higher effects of improving the charge-discharge capacity andreducing the positive electrode resistance are obtained. In the casewhere the surface of the primary particles are coated with LWO in theform of thin films as above, LWO is preferably present on the surface ofthe primary particles of the lithium-metal composite oxide as coatingfilms with a film thickness of 1 to 150 nm.

When the film thickness is less than 1 nm, the coating films may fail tohave sufficient lithium ion conductivity in some cases. Further, whenthe film thickness is over 150 nm, the lithium ion conductivity isreduced, which may result in failure to obtain a higher effect ofreducing the positive electrode resistance in some cases.

However, such coating film may be partially formed on the surface of theprimary particles, and the whole coating film does not need to have afilm thickness in the range of 1 to 150 nm. When the coating film with afilm thickness of 1 to 150 nm is formed at least partially on thesurface of the primary particles, a high effect is obtained.

Further, also in the case where LWO is formed on the surface of theprimary particles is the form of fine particles as well as in the formof a coating thin film, a high effect on the battery characteristics isobtained.

Meanwhile, in the case where the lithium tungstate is non-uniformlyformed between the lithium-metal composite oxide particles, the movementof lithium ions between the lithium-metal composite oxide particles isrendered non-uniform, and therefore a load is applied onto some specificlithium-metal composite oxide particles, which tends to cause adeterioration in cycle characteristics and output characteristics.

Accordingly, the lithium tungstate is preferably uniformly formed alsobetween the lithium-metal composite oxide particles.

Such properties of the surface of the primary particles of thelithium-metal composite oxide can be determined, for example, byobservation using a field emission scanning electron microscope (SEM),and it has been confirmed that the lithium tungstate is formed on thesurface of the primary particles composed of the lithium-metal compositeoxide in the positive electrode active material for nonaqueouselectrolyte secondary batteries of the present invention.

Accordingly, the amount of the lithium tungstate formed is necessary tobe sufficient to reduce the reaction resistance and ensure a sufficientsurface area of the primary particles to contact with the electrolyte.

Lithium is contained in lithium compounds other than the lithiumtungstate present on the surface of the lithium-metal composite oxideparticles (which will be hereinafter referred to as excess amount oflithium) in an amount of preferably 0.08 mass % or less, more preferably0.05 mass % or less, with respect to the total amount of the positiveelectrode active material.

When the excess amount of lithium is 0.08 mass % or less, gas generationat high temperature can be more effectively suppressed.

More specifically, lithium hydroxide and lithium carbonate are present,other than the lithium tungstate, on the surface of the primaryparticles of the lithium-metal composite oxide, and gas generation thatoccurs due to lithium hydroxide and lithium carbonate during storage ofthe battery at high temperature can be more effectively suppressed bycontrolling the excess amount of lithium present on the surface of thelithium-metal composite oxide.

Further, tungsten is contained in the lithium tungstate in an amount of3.0 at % or less, preferably 0.05 to 2.0 at %, with respect to the totalnumber of atoms of Ni, Co, and M contained in the lithium-metalcomposite oxide. This allows the effect of improving the outputcharacteristics to be obtained. Further, the amount of tungsten of 0.05to 2.0 at % allows the amount of LWO formed to be sufficient to reducethe positive electrode resistance and allows it to be sufficient toensure the surface of the primary particles which can contact with theelectrolyte, so that both high charge-discharge capacity and high outputcharacteristics can be further achieved.

When the amount of tungsten is less than 0.05 at %, the effect ofimproving the output characteristics may fail to be sufficientlyobtained, and when the amount of tungsten is over 3.0 at %, the amountof lithium tungstate formed excessively increases to inhibit theconduction of lithium ions between the lithium-metal composite oxide andthe electrolyte, which may result in a reduction in charge-dischargecapacity.

Further, the amount of lithium in the entire positive electrode activematerial increases by the amount of lithium contained in the lithiumtungstate, where the atomic ratio “Li/Me” of the number of atoms of Liwith respect to the sum of the number of atoms of Ni, Co, and M in thepositive electrode active material (Me) is 0.95 to 1.30, preferably 0.97to 1.25, more preferably 0.97 to 1.20. Thus, the ratio Li/Me in thelithium-metal composite oxide particles as a core material is set topreferably 0.95 to 1.25, more preferably 0.95 to 1.20, thereby allowinghigh battery capacity to be obtained and allowing the amount of lithiumthat is sufficient to form a LW compound to be ensured. For obtaininghigher battery capacity, the ratio Li/Me in the entire positiveelectrode active material is set to 0.95 to 1.15, and the ratio Li/Me inthe lithium-metal composite oxide particles is further preferably set to0.95 to 1.10. Here, the core material is the lithium-metal compositeoxide particles excluding the LW compound, and the positive electrodeactive material is obtained therefrom by forming the LW compound on thesurface of the primary particles of the lithium-metal composite oxideparticles.

When the ratio Li/Me is less than 0.95, the reaction resistance of thepositive electrode in the nonaqueous electrolyte secondary battery usingthe obtained positive electrode active material increases, and thus theoutput of the battery decreases. Further, when the ratio Li/Me is over1.30, the initial discharge capacity of the positive electrode activematerial decreases, and the reaction resistance of the positiveelectrode increases as well.

The positive electrode active material of the present invention hasimproved output characteristics by forming lithium tungstate on thesurface of the primary particles of the lithium-metal composite oxide,and the powder characteristics as the positive electrode active materialsuch as particle size and tap density may fall within the range of thoseof commonly used positive electrode active materials.

(2) Method for Producing Positive Electrode Active Material

Hereinafter, a method for producing the positive electrode activematerial for nonaqueous electrolyte secondary batteries of the presentinvention will be described in detail for each step.

[Mixing Step]

The mixing step is a step of obtaining a tungsten-containing mixture ofa lithium-metal composite oxide powder having a layered crystalstructure constituted by primary particles and secondary particlesformed by aggregation of the primary particles, 2 mass % or more ofwater with respect to the lithium-metal composite oxide powder, and atungsten compound or a tungsten compound and a lithium compound, whereinthe molar ratio (which will be hereinafter referred to as Li molarratio) of the total amount of lithium (Li) contained in the water andthe solid tungsten compound or the water, the solid tungsten compound,and the solid lithium compound, with respect to the content of tungsten(W) is 3 to 5.

The amount of the water with respect to the lithium-metal compositeoxide powder in the tungsten-containing mixture (which will behereinafter referred to simply as mixture) is 2 mass % or more. Thisallows tungsten contained in the tungsten compound to penetrate into thevoids between the primary particles communicating with the outside ofthe secondary particles and into the imperfect grain boundaries togetherwith the water and allows a sufficient amount of W to be dispersed onthe surface of the primary particles. The amount of the water needs onlyto be 2 mass % or more, but when the amount of the water is excessivelylarge, the efficiency of the heat treatment in the subsequent stepdecreases, or the elution of lithium from the lithium-metal compositeoxide particles increases, which may result in an excessively high Limolar ratio in the mixture and a deterioration in batterycharacteristics when the positive electrode active material to beobtained is used as a positive electrode of a battery. Therefore, theamount of the water is preferably 20 mass % or less, more preferably 3to 15 mass %, further preferably 3 to 10 mass %.

When the amount of the water falls within the aforementioned range, thepH is raised due to the lithium eluted in the water, and an effect ofsuppressing the elution of excess lithium is exerted. The molar ratio ofCo and M in the lithium-metal composite oxide powder is maintained alsoin the positive electrode active material.

The tungsten compound to be used is preferably soluble in the watercontained in the mixture so as to penetrate to the surface of theprimary particles inside the secondary particles. Thus, the tungstencompound to be used includes a tungsten compound in the form of anaqueous solution.

The tungsten compound present in the form of an aqueous solution needsonly to be present in an amount that allows the penetration to thesurface of the primary particles inside the secondary particles, andtherefore the tungsten compound in solid form may be partially mixed.Further, even if the compound is difficult to dissolve in the water atroom temperature, the compound needs only to be dissolved in the waterwhen heated in the heat treatment. Further, the water in the mixturebecomes alkaline depending on the lithium to be contained, and thereforethe compound may be capable of being dissolved under an alkalinecondition.

In this way, the tungsten compound is not limited as long as it iscapable of being dissolved in the water, but tungsten oxide, lithiumtungstate, ammonium tungstate, and sodium tungstate, for example, arepreferable, and tungsten oxide, lithium tungstate, and ammoniumtungstate with a low possibility of contamination are more preferable,and tungsten oxide and lithium tungstate are further preferable.

The Li molar ratio of this mixture is set to 3.0 or more and 5.0 orless.

Thereby, the proportion of Li₄WO₅ in the positive electrode activematerial to be obtained can be adjusted to 50 to 90 mol %. When the Limolar ratio is less than 3.0, the proportion of Li₄WO falls below 50 mol%, and when the Li molar ratio is over 5.0, the proportion of Li₄WO₅exceeds 90 mol %, and the excess amount of lithium exceeds 0.08 mass %with respect to the total amount of the positive electrode activematerial.

Therefore, for controlling the proportion of Li₄WO₅ and reducing theexcess amount of lithium, the Li molar ratio is preferably less than4.5, more preferably 4.0 or less. Depending on the tungsten compound tobe added, the Li molar ratio may fall below 3.0 in some cases. In such acase, the deficit may be made up by adding a lithium compound, and asthe lithium compound, a water soluble compound such as lithium hydroxide(LiOH) is preferable.

Further, the amount of tungsten contained in the mixture is preferably3.0 at % or less, more preferably 0.05 to 2.0 at %, with respect to thetotal number of atoms of Ni, Co, and M contained in the lithium-metalcomposite oxide powder. Thereby, the amount of tungsten contained in thelithium tungstate in the positive electrode active material is adjustedto the preferable range, and thus both high charge-discharge capacityand high output characteristics of the positive electrode activematerial can be further achieved.

In the mixing step, the mixing may be performed by supplying the watertogether with the tungsten compound so that the content of the water inthe mixture is 2 mass % or more, or the mixing may be performed bysupplying an aqueous solution of the tungsten compound or separatelysupplying the tungsten compound and the water.

Meanwhile, in the lithium-metal composite oxide powder obtained byfiring the metal composite hydroxide or the metal composite oxide andthe lithium compound, an unreacted lithium compound is present on thesurface of the secondary particles and the primary particles.

Therefore, there may be a case where the amount of lithium present inthe water constituting the mixture excessively increases, and thecontrol of the Li molar ratio may be rendered difficult.

Accordingly, a water washing step of washing with water by mixing thelithium-metal composite oxide powder with water to obtain a slurry ispreferably provided before the mixture is obtained, and a solid-liquidseparation step of performing solid-liquid separation after the washingwith water is preferably provided for adjusting the water content.

By providing the water washing step, the amount of lithium present inthe water in the mixture is reduced, so that the control of the Li molarratio can be facilitated.

The water washing conditions in the water washing step need only to besuch that the unreacted lithium compound can be sufficiently reduced,for example, to preferably 0.08 mass % or less, more preferably 0.05mass % or less, with respect to the total amount of the lithium-metalcomposite oxide particles, and when obtaining the slurry, theconcentration of the lithium-metal composite oxide powder contained inthe slurry is preferably set to 200 to 5000 g with respect to 1 L of thewater in stirring.

When the concentration of the lithium-metal composite oxide powder isset to this range, the unreacted lithium compound can be moresufficiently reduced, while the deterioration due to the elution oflithium from the lithium-metal composite oxide particles is suppressed.

The water washing time and the water washing temperature also may be setto ranges so as to allow the unreacted lithium compound to besufficiently reduced. For example, the water washing time is preferablyin the range of 5 to 60 minutes, and the water washing temperature ispreferably in the range of 10 to 40° C.

In the present invention, the step of adding the tungsten compound isnot limited as long as the mixture as described above is obtained, butin the case where the water washing step is provided, the mixing step ispreferably finished after the water washing step. If the mixture isobtained before the water washing step, the tungsten compound is washedaway by the washing with water, and therefore the amount of tungsten inthe mixture may fall short.

Accordingly, in the case where the water washing step is provided, aspecific mixture is preferably obtained by adding the tungsten compoundat least either during the water washing step or after the solid-liquidseparation step.

In the case where the tungsten compound is added during the waterwashing step, the tungsten compound may be added in advance to the waterthat is mixed with the lithium-metal composite oxide powder to obtain anaqueous solution or a suspension, or may be added after obtaining theslurry. Further, for facilitating the control of the total amount oflithium, the tungsten compound is preferably totally dissolved in theslurry in the washing with water. Further, it is preferable to use atungsten compound poorly soluble in water or a compound free fromlithium.

These compounds enable the influence by lithium dissolved from the solidtungsten compound in the mixture to be reduced and the total amount oflithium in the mixture to be easily controlled.

Meanwhile, also in the case of adding the tungsten compound after thesolid-liquid separation step, the tungsten compound may be in the formof either an aqueous solution or a powder. In the case of adding itafter the solid-liquid separation step, the control of the Li molarratio is facilitated, since lithium and tungsten that are removedtogether with liquid component do not exist, and the entire tungstencompound remains in the mixture.

Further, when adding the tungsten compound during the water washingstep, the tungsten compound may be in the form of either an aqueoussolution or a powder, and a uniform mixture is obtained by adding thetungsten compound to the slurry followed by stirring.

In the case of using an aqueous solution of the tungsten compound or awater-soluble compound for the tungsten compound used, the tungstencompound dissolved in the slurry in the solid-liquid separation stepafter the washing with water is removed together with the liquidcomponent of the slurry. However, the amount of tungsten in the mixturecan be made sufficient by the tungsten dissolved in the water in themixture.

Depending on the water washing conditions and the solid-liquidseparation conditions, the amount of tungsten in the mixture is allowedto be stable together with the water content, and therefore theseconditions may be determined by preliminary tests together with the typeand the amount of the tungsten compound to be added.

The total amount of lithium contained in the water and the tungstencompound (which will be hereinafter referred to as total amount oflithium) with respect to the tungsten in the mixture also can bedetermined by preliminary tests in the same manner as in the amount oftungsten.

The amount of tungsten in the mixture when adding the tungsten compoundin the water washing step can be determined by ICP emissionspectroscopy. Further, the amount of lithium contained in the water inthe mixture can be determined from the analysis value of lithium in theliquid component separated by the solid-liquid separation after thewashing with water by ICP emission spectroscopy and the water content.

Meanwhile, the amount of lithium contained in the solid tungstencompound in the mixture can be determined from the tungsten compoundremaining as a solid component by adding the tungsten compound into alithium hydroxide aqueous solution at the same concentration as theliquid component after the washing with water followed by stirring underthe same conditions as in the washing with water, and calculating theamount remaining as a solid component in the mixture from the ratio ofthe tungsten compound remaining as a residue.

Further, the amount of tungsten contained in the mixture when adding thetungsten compound after the solid-liquid separation step can bedetermined from the amount of the tungsten compound to be added.Meanwhile, the total amount of lithium in the mixture may be calculatedas the sum of the amount of lithium contained in the water determinedfrom the analysis value of lithium in the liquid component separated bythe solid-liquid separation after the washing with water by ICP emissionspectroscopy and the water content, and the amount of lithium determinedfrom the tungsten compound to be added, or the amount of the tungstencompound and the amount of the lithium compound.

When adding the tungsten compound as an aqueous solution after thesolid-liquid separation step, the aqueous solution needs to be adjustedas described above so that the water content is preferably not more than20 mass %, and the tungsten concentration is preferably set to 0.05 to 2mol/L.

Thus, a necessary amount of tungsten can be added, while the watercontent in the mixture is regulated. When the water content exceeds 20mass %, the water content may be adjusted again by solid-liquidseparation, where however the Li molar ratio in the mixture needs to bechecked by determining the amount of tungsten and the amount of lithiumin the removed liquid component.

For adjusting the water content in the mixture to 2 mass % or more, themixing after the solid-liquid separation step is preferably performed ata temperature of 50° C. or less. When the temperature exceeds 50° C.,the water content may fall below 2 mass % due to drying during themixing.

The device to be used for the mixing with the tungsten compound is notlimited as long as it is capable of uniform mixing, and a common mixercan be used therefor. For example, the mixing with the tungsten compoundmay be performed sufficiently to an extent such that the shape of thelithium-metal composite oxide particles is not broken, using a shakermixer, a Loedige mixer, a Julia mixer, a V blender, or the like.

In the production method of the present invention, since the compositionof the positive electrode active material to be obtained is composedonly of tungsten that is increased by being added from the lithium-metalcomposite oxide as a base material in the mixing step and lithium thatis added as needed, a known lithium-metal composite oxide having acomposition represented by the formula Li_(z)Ni_(1-x-y)Co_(x)M_(y)O₂(where 0<x≤0.35, 0≤y≤0.35, and 0.95≤z≤1.25 are satisfied, and M is atleast one element selected from Mn, V, Mg, Mo, Nb, Ti, and Al) is usedfor the lithium-metal composite oxide as a base material, for achievinghigh capacity and low reaction resistance.

Meanwhile, in the case of washing with water, the ratio Li/Me (whichcorresponds to z in the formula) decreases due to the elution of lithiumduring the washing with water. Therefore, after the decrement is checkedin advance by preliminary tests, a lithium-metal composite oxide havingan adjusted ratio Li/Me may be used as a material before the washingwith water. The decrement in the ratio Li/Me under the general waterwashing conditions is about 0.03 to 0.08.

Further, also when the water is supplied in the mixing step, lithium iseluted though it is a small amount. Accordingly, z representing theratio Li/Me in the lithium-metal composite oxide as a base material is0.95≤z≤1.30, preferably 0.97≤z≤1.20.

Further, since it is advantageous to increase the contact area with theelectrolyte for improving the output characteristics, it is preferableto use a lithium-metal composite oxide powder which is constituted byprimary particles and secondary particles formed by aggregation of theprimary particles and which has voids and grain boundaries through whichthe electrolyte can penetrate in the secondary particles.

[Heat Treatment Step]

The heat treatment step is a step of heat-treating the produced mixture.

This allows lithium tungstate to be formed from lithium and tungstencontained in the water in the mixture on the surface of the primaryparticles of the lithium-metal composite oxide, so that a positiveelectrode active material for nonaqueous electrolyte secondary batteriesis obtained.

The heat treatment method is not specifically limited as long as the LWOis formed, but the heat treatment is preferably performed at atemperature of 100 to 600° C. in an oxidizing atmosphere such as anoxygen atmosphere or a vacuum atmosphere, while avoiding a reaction withwater and carbonic acid in the atmosphere, for preventing thedeterioration in electrical properties when used as a positive electrodeactive material for nonaqueous electrolyte secondary batteries.

When the heat treatment temperature is less than 100° C., the water isnot sufficiently evaporated, which may result in failure to sufficientlyform the LWO. Meanwhile, when the heat treatment temperature exceeds600° C., the primary particles of the lithium-metal composite oxide arefired, and the tungsten partially forms a solid solution in the layeredstructure of the lithium-metal composite oxide, which may reduce thecharge-discharge capacity of the battery.

Meanwhile, when the tungsten compound contained in the mixture remainsas a solid material, particularly, in the case where the tungstencompound is added as powder after the solid-liquid separation step, therate of temperature increase is preferably set to 0.8 to 1.2° C./minute,until the powder is sufficiently dissolved, for example, until thetemperature exceeds 90° C. Also in the mixing step, the tungstencompound powder is dissolved in the water in the mixture, but settingthe rate of temperature increase as above allows the solid tungstencompound to be sufficiently dissolved during the temperature rise so asto penetrate into the surface of the primary particles inside thesecondary particles.

When dissolving the solid tungsten compound, the heat treatment ispreferably performed in a sealed container so that the water is notvolatilized until the compound is sufficiently dissolved.

The heat treatment time is not specifically limited but is preferably 3to 20 hours, more preferably 5 to 15 hours, for sufficiently evaporatingthe water in the mixture to form the LWO.

(3) Nonaqueous Electrolyte Secondary Battery

The nonaqueous electrolyte secondary battery of the present invention isconstituted by a positive electrode, a negative electrode, a nonaqueouselectrolyte, etc., and constituted by the same components as those ofcommon nonaqueous electrolyte secondary batteries. The embodimentdescribed below is just an example, and the nonaqueous electrolytesecondary battery of the present invention can be implemented byemploying embodiments in which various changes and improvements aremade, using the embodiment shown in this description as a base, based onthe knowledge of those skilled in the art. Further, the applications ofthe nonaqueous electrolyte secondary battery of the present inventionare not specifically limited.

(a) Positive Electrode

Using the positive electrode active material for nonaqueous electrolytesecondary batteries described above, the positive electrode of thenonaqueous electrolyte secondary battery is produced, for example, asfollows.

First, a positive electrode active material in powder form, a conductivematerial, and a binder are mixed, and activated carbon and a solvent forits intended purpose such as a viscosity adjuster are further added, asneeded, and the mixture is kneaded to produce a positive electrodecomposite material paste.

The mixing ratio of each component in the positive electrode compositematerial paste is also an important element to determine the performanceof the nonaqueous electrolyte secondary battery. When the total mass ofthe solid contents of the positive electrode composite materialexcluding the solvent is taken as 100 parts by mass, it is desirablethat the content of the positive electrode active material be 60 to 95parts by mass, the content of the conductive material be 1 to 20 partsby mass, and the content of the binder be 1 to 20 parts by mass, as in apositive electrode of a common nonaqueous electrolyte secondary battery.

The obtained positive electrode composite material paste, for example,is applied to the surface of a current collector made of aluminum foil,followed by drying, to disperse the solvent. In order to enhance theelectrode density, it may be pressed by roll pressing or the like, asneeded.

Thus, a positive electrode in sheet form can be produced. The positiveelectrode in sheet form can be used for producing a battery, forexample, by being cut into a suitable size corresponding to the intendedbattery. However, the method for producing the positive electrode is notlimited to the aforementioned example, and another method may beemployed.

For producing the positive electrode, graphite (such as naturalgraphite, artificial graphite, and expanded graphite) and carbon blackmaterials such as acetylene black and Ketjen Black®, for example, can beused as the conductive material.

The binder serves to hold the active material particles, for whichpolyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE),fluororubber, ethylene propylene diene rubber, styrene butadiene,cellulose resins, and polyacrylic acid, for example, can be used.

As needed, the positive electrode active material, the conductivematerial, and the activated carbon are dispersed, and a solvent todissolve the binder is added to the positive electrode compositematerial. Specifically, an organic solvent such asN-methyl-2-pyrrolidone can be used as the solvent.

(b) Negative Electrode

As the negative electrode, a material formed by applying a negativeelectrode composite material formed into a paste by mixing the binderwith metal lithium, lithium alloy, or the like, or a negative electrodeactive material capable of absorbing and desorbing lithium ions andadding a suitable solvent onto the surface of the current collector madeof a metal foil such as copper, followed by drying and compressing forincreasing the electrode density, as needed, is used.

As the negative electrode active material, a powder material of naturalgraphite, artificial graphite, a fired material of an organic compoundsuch as a phenolic resin, and a carbon material such as cokes, forexample, can be used. In this case, a fluorine-containing resin such asPVDF can be used as the negative electrode binder, as in the positiveelectrode, and an organic solvent such as N-methyl-2-pyrrolidone can beused as the solvent to disperse the active material and the bindertherein.

(c) Separator

A separator is interposed between the positive electrode and thenegative electrode. The separator separates the positive electrode andthe negative electrode from each other and holds the electrolyte. A thinfilm of polyethylene, polypropylene, or the like having a large numberof fine holes can be used as the separator.

(d) Non-Aqueous Electrolyte

The nonaqueous electrolyte is formed by dissolving a lithium salt as asupporting salt in an organic solvent.

As the organic solvent, one selected from cyclic carbonates such asethylene carbonate, propylene carbonate, butylene carbonate, andtrifluoropropylene carbonate, chain carbonates such as diethylcarbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropylcarbonate, ether compounds such as tetrahydrofuran,2-methyltetrahydrofuran, and dimethoxyethane, sulfur compounds such asethyl methyl sulfone and butanesulton, and phosphorus compounds such astriethyl phosphate and trioctyl phosphate can be used alone, or two ormore of these can be mixed for use.

As the supporting salt, LiPF₆, LiBF₄, LiCO₄, LiAsF₆, and LiN(CF₃SO₂)₂,and composite salts of these can be used.

Further, the non-aqueous electrolyte may contain a radical scavenger, asurfactant, a flame retardant, and the like.

(e) Shape and Configuration of Battery

The nonaqueous electrolyte secondary battery of the present inventionconstituted by the positive electrode, the negative electrode, theseparator, and the non-aqueous electrolyte described above can havevarious shapes such as a cylindrical type and a stacked type.

Even if any shape is employed, an electrode body is obtained by stackingthe positive electrode and the negative electrode via the separator, theobtained electrode body is impregnated with the non-aqueous electrolyte,the connection between the positive electrode current collector and thepositive electrode terminal connected to the outside and the connectionbetween the negative electrode current collector and the negativeelectrode terminal connected to the outside are established using leadsfor the current collectors, and the components are sealed in a batterycase, to complete the nonaqueous electrolyte secondary battery.

(f) Characteristics

The nonaqueous electrolyte secondary battery using the positiveelectrode active material of the present invention has high capacity andhigh power.

In particular, the nonaqueous electrolyte secondary battery obtained bya further preferable embodiment using the positive electrode activematerial according to the present invention, for example, when used as apositive electrode of a 2032-type coin battery, has a high initialdischarge capacity of 165 mAh/g or more and a low positive electroderesistance and further has high capacity and high power. Further, italso has high thermostability and excellent safety.

In the method for measuring the positive electrode resistance in thepresent invention, when the frequency dependence of a battery reactionis measured by a common AC impedance method as an electrochemicalevaluation technique, a Nyquist diagram based on the solutionresistance, the negative electrode resistance and the negative electrodecapacity, and the positive electrode resistance and the positiveelectrode capacity is obtained as shown in FIG. 1.

The battery reaction in an electrode is made by the resistancecomponents following charge transfers and the capacity components by anelectric double layer. When these components are shown as an electricalcircuit, a parallel circuit of the resistance and the capacity isobtained, and they are shown as an equivalent circuit in which thesolution resistance and the parallel circuit of the negative electrodeand the positive electrode are connected in series as the entirebattery. The Nyquist diagram determined is subjected to fittingcalculation using the equivalent circuit, and the resistance componentsand the capacity components each can be estimated. The positiveelectrode resistance is equal to the diameter of a semicircle on the lowfrequency side of the Nyquist diagram to be obtained.

From above, the positive electrode resistance can be estimated byperforming the AC impedance measurement on the produced positiveelectrode and subjecting the obtained Nyquist diagram to fittingcalculation using the equivalent circuit.

EXAMPLES

For a secondary battery having a positive electrode using the positiveelectrode active material obtained by the present invention, theperformance (such as initial discharge capacity and positive electroderesistance) was measured.

Hereinafter, the present invention will be specifically described by wayof examples, but the present invention is not limited to these examplesat all.

(Production and Evaluation of Battery)

For evaluating the initial discharge capacity and the positive electroderesistance of the positive electrode active material, a 2032-type coinbattery 1 (which will be hereinafter referred to as coin type battery)shown in FIG. 2 was used.

As shown in FIG. 2, the coin type battery 1 is constituted by a case 2and electrodes 3 housed in the case 2.

The case 2 has a hollow positive electrode can 2 a with one end open anda negative electrode can 2 b arranged in the opening of the positiveelectrode can 2 a, and is configured so that, when the negativeelectrode can 2 b is arranged in the opening of the positive electrodecan 2 a, a space to house the electrodes 3 is formed between thenegative electrode can 2 b and the positive electrode can 2 a. Theelectrodes 3 are constituted by a positive electrode 3 a, a separator 3c, and a negative electrode 3 b, which are stacked to be aligned in thisorder and are housed in the case 2 so that the positive electrode 3 a isin contact with the inner surface of the positive electrode can 2 a, andthe negative electrode 3 b is in contact with the inner surface of thenegative electrode can 2 b.

The case 2 includes a gasket 2 c, and the relative movement between thepositive electrode can 2 a and the negative electrode can 2 b is fixedby the gasket 2 c so that the non-contact state is maintained. Further,the gasket 2 c also has a function of sealing the gap between thepositive electrode can 2 a and the negative electrode can 2 b so as toblock between the inside and the outside of the case 2 air-tightly andliquid-tightly.

The coin type battery 1 shown in FIG. 2 was fabricated as follows.

First, 52.5 mg of the positive electrode active material for nonaqueouselectrolyte secondary batteries, mg of acetylene black, and 7.5 mg ofpolytetrafluoroethylene resin (PTFE) were mixed, followed by pressmolding at a pressure of 100 MPa to a diameter of 11 mm and a thicknessof 100 μm, to produce the positive electrode 3 a. The thus producedpositive electrode 3 a was dried in a vacuum dryer at 120° C. for 12hours.

Using the positive electrode 3 a, the negative electrode 3 b, theseparator 3 c, and the electrolyte, the coin type battery 1 shown inFIG. 2 was produced in a glove box under Ar atmosphere with the dewpoint controlled to −80° C.

As the negative electrode 3 b, a negative electrode sheet in whichgraphite powder with an average particle size of about 20 μm andpolyvinylidene fluoride were applied to a copper foil and which waspunched into a disk shape with a diameter of 14 mm was used.

As the separator 3 c, a polyethylene porous film with a film thicknessof 25 nm was used.

As the electrolyte, an equal mixture (manufactured by TOMIYAMA PURECHEMICAL INDUSTRIES, LTD.) of ethylene carbonate (EC) and diethylcarbonate (DEC) with 1 M LiClO₄ serving as a supporting electrolyte wasused.

The initial discharge capacity and the positive electrode resistanceshowing the performance of the thus produced coin type battery 1 wereevaluated as follows.

The capacity when the coin type battery 1 allowed to stand for about 24hours from the fabrication was charged, with the current density withrespect to the positive electrode set to 0.1 mA/cm², to a cut-offvoltage of 4.3 V after the OCV (Open Circuit Voltage) became stable,followed by a pause for one hour, and was discharged to a cut-offvoltage of 3.0 V was taken as the initial discharge capacity.

The Nyquist plot shown in FIG. 1 is obtained by charging the coin typebattery 1 at a charge potential of 4.1 V and measuring the positiveelectrode resistance using a frequency response analyzer and apotentio-galvanostat (1255B, manufactured by Solartron) by the ACimpedance method. Since the Nyquist plot is shown as the sum ofcharacteristic curves showing the solution resistance, the negativeelectrode resistance and the capacity thereof, and the positiveelectrode resistance and the capacity thereof, fitting calculation wasperformed based on the Nyquist plot using the equivalent circuit tocalculate the value of the positive electrode resistance.

For evaluating the gas generation of the positive electrode activematerial, a laminated cell 4 shown in the schematic illustration of FIG.4 was used.

For producing the laminated cell 4, a positive electrode sheet 5 inwhich a positive electrode active material layer with a mass per unitarea of the positive electrode active material of 7 mg/cm² was formed byapplying a pasted positive electrode active material to an aluminumcurrent collector foil (with a thickness of 0.02 mm) with a conductiveunit connected to the outside left uncoated, followed by drying, wasproduced.

Further, a negative electrode sheet 6 in which a negative electrodeactive material layer with a mass per unit area of the negativeelectrode active material of 5 mg/cm² was formed by applying a pastedcarbon powder (acetylene black) in the same manner as above to a coppercurrent collector foil (with a thickness of 0.02 mm) as a negativeelectrode active material was produced.

A separator 7 constituted by a polypropylene microporous film (with athickness of 20.7 μm and a void fraction density of 43.9%) wasinterposed between the positive electrode sheet 5 and the negativeelectrode sheet 6 produced above to form a laminated sheet. Then, thelaminated sheet was sandwiched by two pieces of aluminum laminatedsheets 8 (with a thickness of 0.05 mm), and three sides of the aluminumlaminated sheets were thermally fused to be sealed, so that thelaminated cell with the configuration as shown in FIG. 4 was assembled.

Thereafter, 260 μl of an electrolyte manufactured by Ube Industries,Ltd. in which LiPF₆ (1 mol/L) and cyclohexyl benzene (2 wt %) weredissolved in a mixed solvent of ethylene carbonate, ethyl methylcarbonate, and dimethyl carbonate (volume ratio 3:3:4) was injected, andthe remaining one side was thermally fused, to produce the laminatedcell 4 shown in FIG. 4 for the gas generation test to evaluate the gasgeneration. The produced laminated cell 4 had a size with a length of 60mm and a width of 90 mm.

(Gas Generation Test)

The produced laminated cell 4 was stored for 12 hours in a thermostaticbath (COSMOPIA) manufactured by Hitachi Appliances, Inc. set at 25° C.

While the laminated cell 4 was housed in the thermostatic bath after thelaminated cell 4 was stored for 12 hours, charge and discharge wereperformed three times in a 0.2 C constant current mode within the rangeof 3.0 to 4.3 V, using a charge-discharge device (HJ1001SD8,manufactured by HOKUTO DENKO CORPORATION). After the charge anddischarge, the laminated cell 4 was charged to 4.6 V in a 1 C constantcurrent mode and then was allowed to stand in a thermostatic bath for 72hours, so that gas was generated in the laminated cell 4.

At this time, the laminated cell 4 was held by being sandwiched betweena pair of plate members (made of stainless steel), and a width of 1 cmfrom the end of the laminated cell was exposed from the pair of platemembers to serve as an exposed portion.

(Evaluation of Amount of Generated Gas)

The tested laminated cell 4 a that had undergone the gas generation test(which will be hereinafter referred to as tested laminated cell) wastaken out of the thermostatic bath, and a mark was made at a width of 1cm from the end of the tested laminated cell 4 a using a permanentmarker.

Thereafter, the tested laminated cell 4 a was placed on a table T of amanual hydraulic press machine PA4 (model number: TB-50H, manufacturedby NPa SYSTEM CO., LTD.) as shown in the schematic illustration of themethod for evaluating the gas generation in FIG. 5, and a rectangularparallelepiped pressing plate (made of stainless steel) serving as apressing member PP was placed on the tested laminated cell 4 a with awidth of 1 cm from the end (an unpressed part UPA with a width L₁, whichis a portion from the marked portion to the end of the tested laminatedcell 4 a) left.

Further, a rectangular parallelepiped measuring plate (made of stainlesssteel) serving as a mounting member MP was arranged on the unpressedpart UPA, and a dial gauge Ga (2A-104, manufactured by CITIZENFINEDEVICE CO., LTD.) was installed on the upper surface of one end(portion placed on the unpressed part) of the measuring plate.

Thereafter, the pressing member PP was pressed by the manual hydraulicpress machine PA as shown in FIG. 5 to apply a pressure of 4 kN to thetested laminated cell 4 a, so that the gases inside the tested laminatedcell 4 a are collected to the unpressed part UPA. The unpressed part UPAwas inflated by the collected gases, and one end of the mounting memberMP moved upward.

Finally, the amount of movement of the one end of the mounting member MPwas measured by reading the value of the dial gauge Ga, to evaluate theamount of the generated gas.

In the present examples, the positive electrode active material, and thesecondary battery, the respective samples of special reagentsmanufactured by Wako Pure Chemical Industries, Ltd. were used forproducing the composite hydroxide.

Example 1

A powder of lithium-metal composite oxide particles represented byLi_(1.030)Ni_(0.82)Co_(0.15)Al_(0.03)O₂ and obtained by a knowntechnique of mixing an oxide powder containing Ni as a main componentand lithium hydroxide followed by firing was used as a base material.The lithium-metal composite oxide powder had an average particle size of12.4 μm and a specific surface area of 0.3 m²/g. The average particlesize was evaluated using a volume integrated average in the laserdiffraction light-scattering method, and the specific surface area wasevaluated using the BET method by nitrogen gas adsorption.

15.6 g of tungsten oxide (WO₃) was added into an aqueous solution inwhich 5.6 g of lithium hydroxide (LiOH) was dissolved in 100 ml of purewater, followed by stirring, to obtain an aqueous solution of a tungstencompound.

Next, 75 g of a lithium-metal composite oxide powder as a base materialwas immersed with the aqueous solution, followed by further stirring for10 minutes, to be sufficiently mixed, and the lithium-metal compositeoxide powder was washed with water at the same time. Thereafter,solid-liquid separation was performed thereon by suction filtrationusing a Buchner funnel, to obtain a tungsten-containing mixture composedof the lithium-metal composite oxide particles, the liquid component,and the tungsten compound.

The mixture was dried, and the water content determined from the massbefore and after the drying was 7.6 mass % with respect to thelithium-metal composite oxide particles.

Further, as a result of analysis by ICP emission spectroscopy, theliquid component had a Li concentration of 2.62 mol/L, the mixture had atungsten content of 0.0039 mol, and the Li molar ratio was 3.9.

The obtained mixture was put into a firing container made of stainlesssteel (SUS), and the temperature was raised in vacuum atmosphere at arate of temperature increase of 2.8° C./minute up to 210° C. for heattreatment for 13 hours, followed by cooling to room temperature in thefurnace.

Finally, a sieve with a mesh opening of 38 μm was applied fordeagglomeration, to obtain a positive electrode active material havinglithium tungstate on the surface of the primary particles.

As a result of analyzing the tungsten content and the ratio Li/Me in theobtained positive electrode active material by ICP emissionspectroscopy, the composition was confirmed to be such that the atomicratio of Ni:Co:Al was 82:15:3, and the tungsten content was 0.5 at %with respect to the total number of atoms of Ni, Co, and M, the ratioLi/Me was 0.994, and the ratio Li/Me in the core material was 0.992.

The ratio Li/Me was determined, using a lithium hydroxide solutioncontaining Li at the same concentration as in the washing with water, byanalyzing a lithium-metal composite oxide powder washed with water inthe same conditions by ICP emission spectroscopy.

[Analysis of Lithium Tungstate and Excess Lithium]

The state of lithium tungstate in the obtained positive electrode activematerial was evaluated by titrating Li eluted from the positiveelectrode active material. Pure water was added to the obtained positiveelectrode active material, the resultant mixture was stirred for acertain time, and them filtrated to obtain a filtrate. Hydrochloric acidwas added to the filtrate while measuring the pH until the point ofneutralization emerged. When the state of a compound containing lithiumeluted at the neutralization point was evaluated, the presence of Li₄WO₅was confirmed in the lithium tungstate, and the proportion of Li₄WO₅contained therein, as calculated, was 60 mol %.

Meanwhile, lithium tungstate was produced by mixing so that W intungsten oxide and Li in lithium hydroxide had the same Li molar ratio(3.9), and the produced lithium tungstate was investigated by X-raydiffraction. As a result, only Li₄WO₅ and Li₂WO₄ were observed.Therefore, it was inferred that, in the lithium tungstate of thepositive electrode active material, the proportion of Li₄WO₅ was 60 mol%, and the proportion of Li₂WO₄ was 40 mol %. Further, the excesslithium was 0.03 mass % with respect to the total amount of the positiveelectrode active material.

[Morphological Analysis of Lithium Tungstate]

The obtained positive electrode active material was embedded into aresin, and cross-section polishing was performed thereon to produce asample for observation. The cross section of the sample was observed bySEM at 5000-fold magnification, and it was confirmed that the sample wasconstituted by primary particles and secondary particles formed byaggregation of the primary particles, fine particles of lithiumtungstate were formed on the surface of the primary particles, and thefine particles had a particle size of 20 to 145 nm.

Further, it was confirmed that 90% of the number of the observedsecondary particles had lithium tungstate formed on the surface of theprimary particles, and the lithium tungstate was uniformly formedbetween the secondary particles.

Further, the vicinity of the surface of the primary particles of theobtained positive electrode active material was observed by atransmission electron microscope (TEM), and it was confirmed thatcoating films of lithium tungstate with a film thickness of 2 to 85 nmwere formed on the surface of the primary particles, and the coatingfilms were the lithium tungstate.

[Evaluation of Battery]

The battery characteristics of the coin type battery 1 shown in FIG. 2having a positive electrode produced using the obtained positiveelectrode active material were evaluated. The positive electroderesistance was shown as a relative value, taking the evaluation value ofExample 1 as 100. The initial discharge capacity was 204.6 mAh/g.

[Evaluation of Gas Generation]

Using the obtained positive electrode active material as a positiveelectrode material, the laminated cell 4 was produced, which wassubjected to a gas generation test, to evaluate the gas generation. Thegas generation was evaluated as a relative value, taking the evaluationof Example 1 as 100.

Hereinafter, for Examples 2 to 6 and Comparative Examples 1 to 5, onlymaterials and conditions changed from those in Example 1 above areshown.

The results of the morphological analysis of the lithium tungstate andthe evaluation values of the initial discharge capacity and the positiveelectrode resistance of Examples 1 to 6 and Comparative Examples 1 to 5measured are shown in Table 1.

Example 2

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained in the same conditions as in Example 1except that 3.5 g of LiOH and 10.5 g of WO₃ were used.

After drying the tungsten-containing mixture after the solid-liquidseparation, the water content determined from the mass before and afterthe drying was 6.8 mass % with respect to the lithium-metal compositeoxide particles.

Further, as a result of analysis by ICP emission spectroscopy, the Liconcentration in the liquid component was 1.74 mol/L, the tungstencontent in the tungsten-containing mixture was 0.0023 mol, and the Limolar ratio was 3.8.

As a result of analyzing the tungsten content and the ratio Li/Me in theobtained positive electrode active material by ICP emissionspectroscopy, the composition was confirmed to be such that the tungstencontent was 0.3 at % with respect to the total number of atoms of Ni,Co, and M, the ratio Li/Me was 0.994, and the ratio Li/Me in the corematerial was 0.992.

Further, as a result of titration analysis of the obtained positiveelectrode active material, the presence of Li₄WO₅ was confirmed in thelithium tungstate, and the proportion of Li₄WO₅ contained in the lithiumtungstate, as calculated, was 60 mol %.

Meanwhile, lithium tungstate was produced by mixing so that W intungsten oxide and Li in lithium hydroxide had the same Li molar ratio(3.8), and the produced lithium tungstate was investigated by X-raydiffraction. As a result, only Li₄WO₅ and Li₂WO₄ were observed.Therefore, it was inferred that, in the lithium tungstate of thepositive electrode active material, the proportion of Li₄WO₅ was 60 mol%, and the proportion of Li₂WO₄ was 40 mol %.

Further, the excess lithium was 0.02 mass % with respect to the totalamount of the positive electrode active material.

The morphological analysis and the evaluation of the lithium tungstatewere performed in the same manner as in Example 1, and the evaluationresults are shown in Table 1 together with the battery characteristics.

Example 3

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained in the same conditions as in Example 1except that 7.0 g of LiOH and 19.3 g of WO₃ were used.

After drying the tungsten-containing mixture after the solid-liquidseparation, the water content determined from the mass before and afterthe drying was 7.3 mass % with respect to the lithium-metal compositeoxide particles.

Further, as a result of analysis by ICP emission spectroscopy, the Liconcentration in the liquid component was 3.19 mol/L, the tungstencontent in the mixture was 0.0046 mol, and the Li molar ratio was 3.8.

As a result of analyzing the tungsten content and the ratio Li/Me in theobtained positive electrode active material by ICP emissionspectroscopy, the composition was confirmed to be such that the tungstencontent was 0.6 at % with respect to the total number of atoms of Ni,Co, and M, the ratio Li/Me was 0.995, and the ratio Li/Me in the corematerial was 0.993.

Further, as a result of titration analysis of the obtained positiveelectrode active material, the presence of Li₄WO₅ was confirmed in thelithium tungstate, and the proportion of Li₄WO₅ contained in the lithiumtungstate, as calculated, was 60 mol %.

Meanwhile, lithium tungstate was produced by mixing so that W intungsten oxide and Li in lithium hydroxide had the same Li molar ratio(3.8), and the produced lithium tungstate was investigated by X-raydiffraction. As a result, only Li₄WO₅ and Li₂WO₄ were observed.Therefore, it was inferred that, in the lithium tungstate of thepositive electrode active material, the proportion of Li₄WO₅ was 60 mol%, and the proportion of Li₂WO₄ was 40 mol %.

Further, the excess lithium was 0.04 mass % with respect to the totalamount of the positive electrode active material.

The morphological analysis and the evaluation of the lithium tungstatewere performed in the same manner as in Example 1, and the evaluationresults are shown in Table 1 together with the battery characteristics.

Example 4

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained in the same conditions as in Example 1except that 300 g of a lithium-metal composite oxide powder serving as abase material was immersed in 400 ml of pure water and was washed withwater, 4.6 g of lithium hydroxide (LiOH) and 1.44 g of tungsten oxide(WO₃) were added thereto after the solid-liquid separation, followed bysufficient mixing using a shaker mixer (TURBULA Type T2C, manufacturedby Willy A. Bachofen AG) at 30° C., to obtain a tungsten-containingmixture, and the temperature was raised at 1° C./minute until thetemperature of the mixture reached 90° C. in the heat treatment step.

The tungsten-containing mixture was dried, and the water contentdetermined from the mass before and after the drying was 7.5 mass % withrespect to the lithium-metal composite oxide particles.

Further, as a result of analysis by ICP emission spectroscopy, the Liconcentration in the liquid component during the solid-liquid separationwas 0.31 mol/L, the tungsten content in the mixture was 0.062 mol, andthe Li molar ratio was 3.2.

As a result of analyzing the tungsten content and the ratio Li/Me in theobtained positive electrode active material by ICP emissionspectroscopy, the composition was confirmed to be such that the tungstencontent was 2.0 at % with respect to the total number of atoms of Ni,Co, and M, the ratio Li/Me was 0.990, and the ratio Li/Me in the corematerial was 0.988.

Further, as a result of titration analysis of the obtained positiveelectrode active material, the presence of Li₄WO was confirmed in thelithium tungstate, and the proportion of Li₄WO contained in the lithiumtungstate, as calculated, was 55 mol %.

Meanwhile, lithium tungstate was produced by mixing so that W intungsten oxide and Li in lithium hydroxide had the same Li molar ratio(3.2), and the produced lithium tungstate was investigated by X-raydiffraction. As a result, only Li₄WO₅ and Li₂WO₄ were observed.Therefore, it was inferred that, in the lithium tungstate of thepositive electrode active material, the proportion of Li₄WO₅ was 55 mol%, and the proportion of Li₂WO₄ was 45 mol %. Further, the excesslithium was 0.03 mass % with respect to the total amount of the positiveelectrode active material.

The morphological analysis and the evaluation of the lithium tungstatewere performed in the same manner as in Example 1, and the evaluationresults are shown in Table 1 together with the battery characteristics.

Example 5

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained in the same conditions as in Example 1except that 300 g of a lithium-metal composite oxide powder serving as abase material was immersed in 400 ml of pure water and was washed withwater, and an aqueous solution in which 31.2 g of lithium tungstate(Li₄WO₅) was dissolved in 400 ml of pure water was added whileperforming suction filtration as it was after the solid-liquidseparation, to obtain a tungsten-containing mixture.

The tungsten-containing mixture was dried, and the water contentdetermined from the mass before and after the drying was 6.4 mass % withrespect to the lithium-metal composite oxide particles.

Further, as a result of analysis by ICP emission spectroscopy, the Liconcentration in the liquid component during the solid-liquid separationafter the addition of the lithium tungstate was 1.36 mol/L, the tungstencontent in the mixture was 0.0065 mol, and the Li molar ratio was 4.0.

As a result of analyzing the tungsten content and the ratio Li/Me in theobtained positive electrode active material by ICP emissionspectroscopy, the composition was confirmed to be such that the tungstencontent was 0.2 at % with respect to the total number of atoms of Ni,Co, and M, the ratio Li/Me was 0.992, and the ratio Li/Me in the corematerial was 0.989.

Further, as a result of titration analysis of the obtained positiveelectrode active material, the presence of Li₄WO₅ was confirmed in thelithium tungstate, and the proportion of Li₄WO₅ contained in the lithiumtungstate, as calculated, was 75 mol %.

Meanwhile, lithium tungstate was produced by mixing so that W intungsten oxide and Li in lithium hydroxide had the same Li molar ratio(4.0), and the produced lithium tungstate was investigated by X-raydiffraction. As a result, only Li₄WO and Li₂WO₄ were observed.Therefore, it was inferred that, in the lithium tungstate of thepositive electrode active material, the proportion of Li₄WO₅ was 75 mol%, and the proportion of Li₂WO₄ was 25 mol %.

Further, the excess lithium was 0.02 mass % with respect to the totalamount of the positive electrode active material.

The morphological analysis and the evaluation of the lithium tungstatewere performed in the same manner as in Example 1, and the evaluationresults are shown in Table 1 together with the battery characteristics.

Example 6

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained in the same manner as in Example 1except that a powder of lithium-metal composite oxide particlesrepresented by Li_(1.175)Ni_(0.34)Co_(0.33)Mn_(0.33)O₂, having anaverage particle size of 5.6 μm and a specific surface area of 0.7 m²/g,and obtained by a known technique of mixing an oxide powder constitutedby Ni, Co, and Mn with lithium hydroxide followed by firing was used asa base material. The mixture after the solid-liquid separation wasdried, and the water content determined from the mass before and afterthe drying was 7.8 mass % with respect to the lithium-metal compositeoxide particles.

Further, as a result of analysis by ICP emission spectroscopy, the Liconcentration in the liquid component was 2.24 mol/L, the tungstencontent in the mixture was 0.0039 mol, and the Li molar ratio was 3.8.

As a result of analyzing the tungsten content and the ratio Li/Me in theobtained positive electrode active material by ICP emissionspectroscopy, the composition was confirmed to be such that the tungstencontent was 0.5 at % with respect to the total number of atoms of Ni,Co, and M, the ratio Li/Me was 1.146, and the ratio Li/Me in the corematerial was 1.144.

Further, as a result of titration analysis of the obtained positiveelectrode active material, the presence of Li₄WO₅ and Li₂WO₄ wasconfirmed in the lithium tungstate, and as a result of calculating theproportion of Li₂WO₄ contained in the lithium tungstate, it was inferredthat the proportion of Li₄WO₅ was 60 mol %, and the proportion of Li₂WO₄was 40 mol %.

Further, the excess lithium was 0.02 mass % with respect to the totalamount of the positive electrode active material.

The morphological analysis and the evaluation of the lithium tungstatewere performed in the same manner as in Example 1, and the evaluationresults are shown in Table 1 together with the battery characteristics.

Comparative Example 1

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained in the same conditions as in Example 1except that the washing with water was not performed in the aqueoussolution of the tungsten compound but in pure water.

The ratio Li/Me in the obtained positive electrode active material, asanalyzed by ICP emission spectroscopy, was 0.991. The excess lithium was0.03 mass % with respect to the total amount of the positive electrodeactive material.

The morphological analysis and the evaluation of the lithium tungstatewere performed in the same manner as in Example 1, and the evaluationresults are shown in Table 1 together with the battery characteristics.

Comparative Example 2

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained in the same conditions as in Example 1except that 9.5 g of LiOH and 15.6 g of WO₃ were used.

After drying the tungsten-containing mixture after the solid-liquidseparation, the water content determined from the mass before and afterthe drying was 7.6 mass % with respect to the lithium-metal compositeoxide particles.

Further, as a result of analysis by ICP emission spectroscopy, the Liconcentration in the liquid component was 4.22 mol/L, the tungstencontent in the mixture was 0.0039 mol, and the Li molar ratio was 6.3.

As a result of analyzing the tungsten content and the ratio Li/Me in theobtained positive electrode active material by ICP emissionspectroscopy, the composition was confirmed to be such that the tungstencontent was 0.5 at % with respect to the total number of atoms of Ni,Co, and M, the ratio Li/Me was 0.996, and the ratio Li/Me in the corematerial was 0.993.

Further, as a result of titration analysis of the obtained positiveelectrode active material, the presence of Li₄WO₅ was confirmed in thelithium tungstate, and the proportion of Li₄WO₅ contained in the lithiumtungstate, as calculated, was 95 mol %.

Meanwhile, lithium tungstate was produced by mixing so that W intungsten oxide and Li in lithium hydroxide had the same Li molar ratio(6.3), and the produced lithium tungstate was investigated by X-raydiffraction. As a result, only Li₄WO₅ was observed. Therefore, it wasinferred that, in the lithium tungstate of the positive electrode activematerial, the proportion of Li₄WO₅ was 95 mol %, and the proportion ofLi₂WO₄ was 0 mol %.

Further, the excess lithium was 0.08 mass % with respect to the totalamount of the positive electrode active material.

The morphological analysis and the evaluation of the lithium tungstatewere performed in the same manner as in Example 1, and the evaluationresults are shown in Table 1 together with the battery characteristics.

Comparative Example 3

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained in the same conditions as in Example 1except that 4.0 g of LiOH and 15.6 g of WO₃ were used.

After drying the tungsten-containing mixture after the solid-liquidseparation, the water content determined from the mass before and afterthe drying was 7.6 mass % with respect to the lithium-metal compositeoxide particles.

Further, as a result of analysis by ICP emission spectroscopy, the Liconcentration in the liquid component was 1.95 mol/L, the tungstencontent in the mixture was 0.0039 mol, and the Li molar ratio was 2.9.

As a result of analyzing the tungsten content and the ratio Li/Me in theobtained positive electrode active material by ICP emissionspectroscopy, the composition was confirmed to be such that the tungstencontent was 0.5 at % with respect to the total number of atoms of Ni,Co, and M, the ratio Li/Me was 0.994, and the ratio Li/Me in the corematerial was 0.992.

Further, as a result of titration analysis of the obtained positiveelectrode active material, the presence of Li₄WO₅ was confirmed in thelithium tungstate, and the proportion of Li₄WO₅ contained in the lithiumtungstate, as calculated, was 35 mol %.

Meanwhile, lithium tungstate was produced by mixing so that W intungsten oxide and Li in lithium hydroxide had the same Li molar ratio(2.9), and the produced lithium tungstate was investigated by X-raydiffraction. As a result, only Li₄WO₅ and Li₂WO₄ were observed.Therefore, it was inferred that, in the lithium tungstate of thepositive electrode active material, the proportion of Li₄WO₅ was 35 mol%, and the proportion of Li₂WO₄ was 65 mol %.

Further, the excess lithium was 0.02 mass % with respect to the totalamount of the positive electrode active material.

The morphological analysis and the evaluation of the lithium tungstatewere performed in the same manner as in Example 1, and the evaluationresults are shown in Table 1 together with the battery characteristics.

Comparative Example 4

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained in the same conditions as in Example 1except that the washing with water was not performed in the aqueoussolution of the tungsten compound but in pure water, followed bysolid-liquid separation and drying, and 15.1 g of lithium tungstate(Li₄WO₅) was added after the drying, followed by sufficient mixing andstirring using a shaker mixer (TURBULA Type T2C, manufactured by WillyA. Bachofen AG), and heat treatment. After the solid-liquid separationand the drying, the water content was less than 1.0 mass %.

As a result of analyzing the tungsten content and the ratio Li/Me in theobtained positive electrode active material by ICP emissionspectroscopy, the composition was confirmed to be such that the tungstencontent was 0.5 at % with respect to the total number of atoms of Ni,Co, and M, the ratio Li/Me was 0.992, and the ratio Li/Me in the corematerial was 0.991.

Further, as a result of titration analysis of the obtained positiveelectrode active material, the presence of Li₄WO₅ was confirmed in thelithium tungstate, the proportion of Li₄WO₅ contained in the lithiumtungstate, as calculated, was 98 mol %, and the excess lithium was 0.03mass % with respect to the total amount of the positive electrode activematerial.

From the observation by SEM and TEM, it was confirmed that the lithiumtungstate deposited only on the surface of the positive electrode activematerial particles and was not present on the surface of the primaryparticles thereinside.

The morphological analysis and the evaluation of the lithium tungstatewere performed in the same manner as in Example 1, and the evaluationresults are shown in Table 1 together with the battery characteristics.

Comparative Example 5

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained in the same manner as in Example 6except that the washing with water was performed while the aqueoussolution of the tungsten compound was not used and was changed to purewater.

The ratio Li/Me in the obtained positive electrode active material, asanalyzed by ICP emission spectroscopy, was 1.138.

The excess lithium was 0.04 mass % with respect to the total amount ofthe positive electrode active material.

The morphological analysis and the evaluation of the lithium tungstatewere performed in the same manner as in Example 1, and the evaluationresults are shown in Table 1 together with the battery characteristics.

TABLE 1 W concentration Compound on surface of Water in positive primaryparticles Initial content in Li electrode proportion Particle Filmdischarge Positive mixture molar active material of Li₄WO₅ Excess Lisize thickness capacity electrode Gas [mass %] ratio [at %] [mol %][mass %] Form [nm] [nm] [mAh/g] resistance generation Example 1 7.6 3.90.5 60 0.03 Thin film + 20-145 2-85 204.6 100 100 Fine particles Example2 6.8 3.8 0.3 60 0.02 Thin film — 1-65 205.5 116 95 Example 3 7.3 3.80.6 60 0.04 Thin film + 30-165 2-90 200.0 108 110 Fine particles Example4 7.5 3.2 2.0 55 0.03 Thin film + 20-185  2-120 199.8 112 120 Fineparticles Example 5 6.4 4.0 0.2 75 0.02 Thin film — 2-70 206.2 108 101Example 6 7.8 3.8 0.5 60 0.02 Thin film + 20-140 2-70 170.1 70 65 Fineparticles Comparative — — 0.0 — 0.03 — — — 198.2 235 62 Example 1Comparative 7.6 6.3 0.5 95 0.08 Thin film + 20-160 2-90 199.3 105 157Example 2 Fine particles Comparative 7.6 2.9 0.5 35 0.02 Thin film +20-155 2-85 201.7 134 83 Example 3 Fine particles Comparative <1.0  4.00.5 98 0.03 — — — 199.7 157 169 Example 4 Comparative — — 0.0 — 0.04 — —— 158.9 175 38 Example 5

[Evaluation]

As is obvious from Table 1, the positive electrode active materials ofExamples 1 to 6 were produced according to the present invention andtherefore formed batteries having low positive electrode resistance,high initial discharge capacity as compared with Comparative Examples 1and 5 in which the lithium tungstate was not formed, and excellentcharacteristics.

Further, an example of the cross-sectional observation results of thepositive electrode active material obtained in the examples of thepresent invention by a scanning microscope is shown in FIG. 3, where itwas confirmed that the obtained positive electrode active material wasconstituted by primary particles and secondary particles formed byaggregation of the primary particles, and fine particles containinglithium tungstate were formed on the surface of the primary particles.The fine particles containing lithium tungstate are shown by arrows inFIG. 3.

Since no lithium tungstate was formed on the surface of the primaryparticles, Comparative Example 1 has considerably high positiveelectrode resistance, and thus it is difficult to meet the requirementto enhance the power.

In Comparative Examples 2 and 3, the amount of tungsten with respect tothe number of atoms of Ni, Co, and M contained in the positive electrodeactive material was almost the same as in Example 1. However, since theratio of Li₄WO₅ was large in Comparative Example 2, the gas generationwas larger, though the positive electrode resistance was almost the sameas in Examples. Meanwhile, since the ratio of Li₄WO₅ was small inComparative Example 3, the positive electrode resistance was high,though the gas generation was small.

In Comparative Example 4, since the mixing with the tungsten compoundwas performed in a dry state, lithium tungstate was not formed on thesurface of the primary particles inside the secondary particles, and thepositive electrode resistance was high. Further, it is seen that, sincelithium tungstate was Li₄WO₅, the gas generation was also large.

In Comparative Example 5, since lithium tungstate was not formed on thesurface of the primary particles, the positive electrode resistance wasconsiderably high, and thus it is difficult to meet the requirement toenhance the power.

The nonaqueous electrolyte secondary battery of the present invention issuitable for power sources of small portable electronic devices (such aslaptop personal computers and mobile phone terminals) that constantlyrequire high capacity and is suitable for batteries for electric carsthat require high power.

Further, the nonaqueous electrolyte secondary battery of the presentinvention has excellent safety and allows size reduction and powerenhancement, and therefore it is suitable as a power source for electriccars where there is a restriction on the mounting space. The presentinvention can be used not only as a power source for electric cars whichare purely driven by electric energy but also as a power source forso-called hybrid vehicles that is used in combination with a combustionengine such as a gasoline engine and a diesel engine.

REFERENCE SIGNS LIST

-   1: Coin type battery-   2: Case-   2 a: Positive electrode can-   2 b: Negative electrode can-   2 c: Gasket-   3: Electrode-   3 a: Positive electrode-   3 b: Negative electrode-   3 c: Separator-   4: Laminated cell-   4 a: Laminated cell after gas generation test-   5: Positive electrode sheet-   6: Negative electrode sheet-   7: Separator-   8: Aluminum laminated sheet-   PA: Manual hydraulic press machine-   UPA: Unpressed part-   L₁: Width (of unpressed part)-   PP: Pressing member-   MP: Mounting member-   Ga: Dial gauge-   T: Table

1. A method of producing a positive electrode active material fornonaqueous electrolyte secondary batteries, comprising: a mixing step ofobtaining a tungsten-containing mixture of: a lithium-metal compositeoxide powder represented by a general formula:Li_(z)Ni_(1-x-y)Co_(x)M_(y)O₂ (where 0<x≤0.35, 0≤y≤0.35, and 0.95≤z≤1.30are satisfied, and M is at least one element selected from Mn, V, Mg,Mo, Nb, Ti, and Al) and having a layered crystal structure constitutedby primary particles and secondary particles formed by aggregation ofthe primary particles; 2 mass % or more of water with respect to thelithium-metal composite oxide powder; and a tungsten compound or atungsten compound and a lithium compound, the tungsten-containingmixture having a molar ratio of a total amount of lithium contained inthe water and the tungsten compound as a solid component, or in thewater, and the tungsten compound and the lithium compound as a solidcomponent of 3 to 5, with respect to an amount of tungsten containedtherein; and a heat treatment step of heating the obtainedtungsten-containing mixture to form lithium tungstate on a surface ofthe primary particles of the lithium-metal composite oxide.
 2. Themethod of producing a positive electrode active material for nonaqueouselectrolyte secondary batteries according to claim 1, furthercomprising, prior to the mixing step: a water washing step of washingthe lithium-metal composite oxide powder with water by mixing thelithium-metal composite oxide powder with the water to form a slurry;and a solid-liquid separation step of subjecting the slurry tosolid-liquid separation subsequently to the water washing step.
 3. Themethod of producing a positive electrode active material for nonaqueouselectrolyte secondary batteries according to claim 2, wherein thelithium-metal composite oxide powder is contained in the slurry at aconcentration of 200 to 5000 g per 1 L of water.
 4. The method ofproducing a positive electrode active material for nonaqueouselectrolyte secondary batteries according to claim 2, wherein thetungsten compound is added at least during the water washing step orafter the solid-liquid separation step to obtain the tungsten-containingmixture.
 5. The method of producing a positive electrode active materialfor nonaqueous electrolyte secondary batteries according to claim 4,wherein in the water washing step, the lithium-metal composite oxidepowder is mixed with an aqueous solution of the tungsten compound toform the slurry.
 6. The method of producing a positive electrode activematerial for nonaqueous electrolyte secondary batteries according toclaim 4, wherein the tungsten compound is in powder form.
 7. The methodof producing a positive electrode active material for nonaqueouselectrolyte secondary batteries according to claim 1, wherein the heattreatment is performed at 100 to 600° C.
 8. The method of producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to claim 1, wherein an amount of tungsten containedin the tungsten-containing mixture is adjusted to 0.05 to 2.0 at % withrespect to the total number of atoms of Ni, Co, and M contained in thelithium-metal composite oxide powder.
 9. A positive electrode activematerial for nonaqueous electrolyte secondary batteries composed of alithium-metal composite oxide powder having a layered crystal structureconstituted by primary particles and secondary particles formed byaggregation of the primary particles, wherein the positive electrodeactive material is represented by a general formula:Li_(z)Ni_(1-x-y)Co_(x)M_(y)W_(a)O_(2+α) (where 0<x≤0.35, 0≤y≤0.35,0.95≤z≤1.30, 0<a≤0.03, and 0≤α≤0.15 are satisfied, and M is at least oneelement selected from Mn, V, Mg, Mo, Nb, Ti, and Al), and has lithiumtungstate on a surface of the primary particles of the lithium-metalcomposite oxide, and Li₄WO₅ is contained in the lithium tungstate at aproportion of 50 to 90 mol %.
 10. The positive electrode active materialfor nonaqueous electrolyte secondary batteries according to claim 9,wherein lithium contained in a lithium compound other than the lithiumtungstate present on the surface of the lithium-metal composite oxide isin an amount of 0.08 mass % or less with respect to a total amount ofthe positive electrode active material.
 11. The positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto claim 9, wherein tungsten is contained in the lithium tungstate in anamount of 0.05 to 2.0 at % in terms of the number of W atoms withrespect to the total number of atoms of Ni, Co, and M contained in thelithium-metal composite oxide.
 12. The positive electrode activematerial for nonaqueous electrolyte secondary batteries according toclaim 9, wherein the lithium tungstate is present on the surface of theprimary particles of the lithium-metal composite oxide as fine particleshaving a particle size of 1 to 200 nm.
 13. The positive electrode activematerial for nonaqueous electrolyte secondary batteries according toclaim 9, wherein the lithium tungstate is present on the surface of theprimary particles of the lithium-metal composite oxide as a coating filmhaving a film thickness of 1 to 150 nm.
 14. The positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto claim 9, wherein the lithium tungstate is present on the surface ofthe primary particles of the lithium-metal composite oxide in both formsof fine particles having a particle size of 1 to 200 nm and a coatingfilm having a film thickness of 1 to 150 nm.
 15. A nonaqueouselectrolyte secondary battery comprising: a positive electrodecomprising the positive electrode active material for nonaqueouselectrolyte secondary batteries according to claim
 9. 16. The method ofproducing a positive electrode active material for nonaqueouselectrolyte secondary batteries according to claim 1, wherein an amountof the water in the tungsten-containing mixture is 3 to 15 mass % withrespect to the lithium-metal composite oxide powder.